Different parts of the world provide television images on a video screen and the accompanying sound by using systems which are not compatible with one another. The United States was the first to adopt specifications for a television system. The system adopted is designated as "NTSC". Its specifications are not as rigorous in providing a clear image as the specifications adopted for the television systems in other parts of the world. In most of Europe, the system adopted is designated as "PAL". Since it was adopted after the adoption of the NTSC system, its specifications are more rigorous than the NTSC system and it provides a clearer image than the NTSC system.
In each of the NTSC and PAL television systems, a pixel clock frequency is provided for transmitting the television signals. The pixel clock signals for each of the NTSC and PAL television systems are different from the pixel clock signals for the other television systems. Furthermore, each of the different television systems has a number of different pixel clock frequencies depending upon the mode or use of the television system. For example, each of the NTSC and PAL television systems requires pixel clock signals at an individual frequency depending upon the mode or use of the system in applications involving square pixels or in digital video or studio applications.
In addition to requiring a pixel clock frequency, each of the NTSC and PAL systems provides a sub-carrier frequency at a particular frequency below the pixel clock frequency. The sub-carrier frequency for each of the NTSC and PAL systems is different from the sub-carrier frequencies for the other system. However, the sub-carrier frequency for each of the NTSC and
systems remains the same regardless of the mode provided (square pixels, studio or digital). The signals at the sub-carrier frequency provide the color burst at the beginning of a line and carry the chrominance information.
It would be desirable to provide a system which generates signals in each of the NTSC and PAL formats and in any of the different modes (square pixel, studio and digital) and which provides sub-carrier signals at the proper frequency regardless of the format and mode used. However, in spite of the existence of such desirability for some time and the realization by the television and data processing industries of the existence of such desirability, no one has been able to provide a system which is able to fulfill this desirability by providing a system with the required universality.
Another problem has existed in the television industry for some time. When luminance and chrominance signals providing color information are produced, the amplitude of the chrominance signals sometimes rises above a level which the receiving system is not equipped to properly handle or falls to or below a level at which video timing information may not be correctly recovered. This problem has existed for some time without a satisfactory resolution even through the appreciation of the problem, and the need or desirability for its resolution, have existed for such time.